The present invention relates to a preassembled fastener and washer assembly.
Prior art assemblies of this type have been generally formed by assembling the washer to the fastener followed by a thread rolling step. While this technique and assembly have proved effective with some applications, there are instances where this method and assembly are unsuitable. For example, when used with a domed or stand-off type washer this technique prevents that portion of the fastener surrounded by the washer from being threaded. In those applications where the assembly is to be used with a thin workpiece, the inner periphery of the aperture may have a tendency to be drawn up underneath the washer. In the prior art assemblies, absence of threads in this region may lead to the stripping of the threads in the workpiece or other failure of the joint.
Related applications include U.S. Ser. No. 932,744 and Ser. No. 6,956. In an effort to overcome the problems created by an absence of threads in the upper region of the fastener, the teachings of the above-noted application Ser. No. 932,744 were applied. That is, attempts were made to assemble the washer and fastener after thread rolling. In this particular application, however, it is desirable to case harden the threads of the fastener while maintaining the flexibility of the washer. In order to do this, a heavy copper plate must be applied to the washer prior to heat treating and stripped off after the heat treatment. In addition to the cost involved in such a process, problems include the fact that once the copper is stripped away, the inside diameter of the washer is enlarged and occasionally permits the washer to become disassembled from the fastener. A further attempt was made to overcome these problems by heat treating the fastener first and then assembling the washer. However, problems arose in this instance due to the fact that the annular ring on the fastener had been case hardened and when the washer was pushed thereover, a sliver generally broke off the ring, again, allowing the washer to fall off.
It is therefore an object of the present invention to provide a domed washer and fastener assembly in which the fastener is threaded in that region which is surrounded by the washer.
It is a further object of the present invention to provide an assembly which may be put together following heat treatment thereby eliminating the expensive copper plating method previously utilized.
These and other objects of the invention are accomplished by providing a washer and fastener assembly in which the fastener has an unthreaded region which includes an interrupted annular ledge. The washer has an aperture for receiving the fastener with a diameter which is greater than the thread crest diameter but less than the diameter of the interrupted annular ledge. The inner periphery of the aperture has been coined in order to reduce the thickness of material in this region. This increases the flexibility of the inner periphery of the washer allowing it to be sprung over the interrupted annular ledge by merely pushing the washer thereover. The interrupted annular ledge is formed as a knurled ring. In this fashion, if there is any tendency of the washer to wipe away material from this hardened knurled ring, only minute particles will be removed, thereby insuring sufficient interference between the coined edge and the annular ledge to guarantee retention of the washer on the fastener.
These and other objects, advantages and features of the invention will be more fully understood by reading the following specification in conjunction with the drawings which form a part thereof.